Low heat rejection cylinder heads offer numerous advantages in the performance of internal combustion engines, and particularly diesel engine exhaust and air systems. These advantages include reduced cooling system burdens as well as improved engine performance, reliability, durability and fuel economy. Much of the benefit obtained is a result of the synergistic effect one design feature has on the other. For example, the cylinder heads which port the high temperature exhaust gases from the combustion chamber to an exhaust manifold are generally water cooled. To the extent that the amount of heat from the exhaust gases can be reduced, the cooling requirements are likewise reduced which can lead to advantages of lower capacity, and lower cost, cooling systems.
Further, given that the heat transfer of the exhaust gases given up to the cylinder head can be reduced, the exhaust gases themselves will be hotter and the increased energy therein can be used to good effect in turbo-charging or otherwise preconditioning the engine intake air to be used for combustion.
Heretofore, the state of the art has been to incorporate cast-in- place stainless steel heat shields in the exhaust ports of the cylinder head. The heat shields provided thermal insulating air gaps between the hot exhaust gases exiting the combustion chamber and the surface of the cast cylinder head wall defining the exhaust port cavities containing the heat shields. The opposite side of this cast wall is in contact with coolant circulating through the cylinder head. By reducing heat loss from the hot gases in the exhaust ports, more heat energy is available in the exhaust gases, where it can be productively used by a turbocharger, for example.
In the aforementioned known construction, the exhaust shields served to create an air gap between the outer shield surface and the water cooled port wall of the cylinder head casting, thereby reducing the amount of heat transferred from the exhaust gas to the cylinder head and thereby to the cylinder head coolant. By reducing the amount of heat transferred to the coolant, the engine's cooling system burden (i.e., total engine heat rejected to the coolant) has been typically reduced by as much as 15-23%. Further benefits result from the fact that by shielding the exhaust gases from the cylinder head casting, more exhaust gas heat energy is retained for utilization in the turbo-charger which increases the overall thermal efficiency of the engine.
Using the cast-in-place method, the cast stainless steel exhaust shield is inserted into the cylinder head mold before the iron is poured. As the iron is poured, a thin layer of sand around the outside of the shield serves to maintain a space between the adjacent interior wall of the cylinder head and the shield. At certain areas of the shield, the iron actually fuses to the shield forming a diffusion bond. This bond results in a permanent jointure between the two pieces. When the casting is cooled, the sand is removed and the air gap remains, covering as much as 90% or more of the surface area of the exhaust gas exit passage through the cylinder head (exhaust port).
The cast-in-place method is superior to a shield that is inserted after the casting process in several ways. Space utilization is excellent since assembly clearances are not needed. Also, cylinder head machining is greatly reduced because the cylinder head to shield mating surfaces are integrally bonded at the desired interface junctures. This forms a completed assembly directly out of the mold.
The cylinder head's low heat rejection function centers around the stainless steel exhaust shield. The term "shield" is used herein because the part's function is to shield the cylinder head water jacket system from unwanted exhaust gas heat. This function requires a material of superior high temperature strength and corrosion resistance. Because the air gap reduces the heat transfer from the exhaust gases, the shield temperature will approach exhaust gas temperatures, which typically are at about or slightly in excess of 480.degree. Centigrade (900.degree. F.) in a two-stroke diesel engine. AISI 347 stainless steel is a known suitable material for this heat shield application.
The shield itself is a casting, being produced by a vacuum-assisted casting process allowing various materials to be cast with very thin walls, i.e., in the order of 0.178 centimeters (0.070 inches) and improved dimensional stability. Such a process is described in U.S. Pat. No. 4,340,108.
The process for casting the shield in place is similar to normal gravity sand casting, with principal variations as described below. After the shield is cast, a machining operation finishes the end of the shield, i.e., that which connects to the exhaust manifold, for a tight, sliding, interengaging-type fit with a flange seal to be incorporated between the exhaust manifold gasket-cylinder head interface. A slip fit sealing arrangement of this type is generally shown in FIG. 6. Once machined, the shields may be plated to provide an enhanced diffusion bond with the cast iron. The shield is then placed into a core box. The cold box core operation locates the shield and blows the desired amount of sand around the shield to form the air gap and fill in the interior of the shield.
In engines where each combustion chamber has two or more exhaust ports, particularly where they are diametrically opposed from one another, it is not uncommon to use two shields and to make up a pair of exhaust port cores containing the shields as a single core, thereby forming the exhaust passage for one cylinder position in the cylinder head. At this point, a graphite-based refractory coating (core wash) is applied to the core to inhibit bonding at certain areas of the shields. Core washes are normally applied to the cores to facilitate sand release from the resultant iron surface.
Upon completing the casting of the cylinder head, the core sand is removed, thereby providing, among other things, an air gap between the heat shield and cylinder head interior. A flange seal may thereafter be mounted on the heat shield at the end nearest the exhaust gas outlet.